{"entity":{"id":"her2-ihc-1-plus","kind":"biomarker","name":"HER2 IHC 1+","aka":["HER2 IHC 1+","HER2 1+","IHC 1+","HER2 1+ breast cancer"],"tldr":"IHC 1+ is faint, incomplete HER2 staining. It was called HER2-negative for twenty years; since 2022 it is the larger half of HER2-low, which trastuzumab deruxtecan treats in breast cancer.","summary":"1+ is incomplete, faint or barely perceptible membrane staining in more than 10 percent of tumour cells. Under the ASCO/CAP guideline it is HER2-negative for trastuzumab purposes, and no gene test is needed. The DESTINY-Breast04 trial (2022) and DESTINY-Breast06 (2025) defined HER2-low as IHC 1+ or IHC 2+/ISH-, and trastuzumab deruxtecan's US label adopts that wording, so a 1+ result now selects for an antibody-drug conjugate that the parent antibody trastuzumab would not be given for. About half of breast cancers once labelled HER2-negative fall into 1+.","asOf":"2026-09-23","links":[{"label":"ENHERTU prescribing information (DailyMed)","url":"https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=7e67e73e-ddf4-4e4d-8b50-09d7514910b6"},{"label":"ASCO/CAP HER2 testing in breast cancer guideline, 2018 focused update (Wolff et al., J Clin Oncol)","url":"https://doi.org/10.1200/JCO.2018.77.8738"}],"tags":["biomarker","her2"],"related":["her2-low-ihc","her2-ihc-0","her2-ihc-2-plus"],"cancers":["breast-cancer","her2-low-metastatic-breast-cancer","breast-hr-positive"],"sections":[],"technologies":[],"targets":[],"drugs":["trastuzumab-deruxtecan","her2-testing-assays"],"companies":[],"institutions":[],"pathways":[],"terms":["her2-low","ihc"],"trials":[],"people":[],"bottlenecks":[],"keyPapers":[],"journals":[],"dependsOn":[],"notes":[],"target":"her2","measurement":"ihc-score","scoringRule":{"text":"1+: incomplete membrane staining that is faint or barely perceptible in more than 10 percent of tumour cells; HER2-negative under ASCO/CAP, HER2-low for trastuzumab deruxtecan.","quote":"HER2-low (IHC 1+ or IHC 2+/ISH-) breast cancer, as determined by an FDA-authorized test","source":"https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=7e67e73e-ddf4-4e4d-8b50-09d7514910b6","sourceLabel":"ENHERTU prescribing information; scoring band from the ASCO/CAP 2018 guideline"},"thresholds":[{"value":"IHC 1+ (HER2-low)","drugId":"trastuzumab-deruxtecan","cancerId":"her2-low-metastatic-breast-cancer","regulator":"FDA","source":"https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=7e67e73e-ddf4-4e4d-8b50-09d7514910b6","quote":"Select patients for treatment of unresectable or metastatic HER2-low (IHC 1+ or IHC 2+/ISH-) or HER2-ultralow (IHC 0 with membrane staining) breast cancer with ENHERTU based on HER2 expression","status":"current"}],"tests":[],"assays":["her2-herceptest","her2-pathway-4b5"],"companionDiagnostics":[{"device":"PATHWAY anti-Her2/neu (4B5) Rabbit Monoclonal Primary Antibody","maker":"Ventana Medical Systems (Roche)","drugs":["trastuzumab-deruxtecan"],"indication":"Breast Cancer - Tissue","pma":"P990081/S047 (09/30/2022)","source":"https://www.fda.gov/medical-devices/in-vitro-diagnostics/list-cleared-or-approved-companion-diagnostic-devices-in-vitro-and-imaging-tools"}],"forPatient":"A 1+ result still counts as HER2-negative for trastuzumab, but in breast cancer it now means HER2-low, and trastuzumab deruxtecan is on label once the cancer has spread and earlier chemotherapy or hormone therapy has stopped working. If your report is older than 2022 the 1+ score may not have been mentioned to you; it is worth asking."},"route":"/biomarkers/her2-ihc-1-plus/","neighbours":{"biomarker":[{"id":"her2-ihc-0","kind":"biomarker","name":"HER2 IHC 0 (HER2-negative, including ultralow)","route":"/biomarkers/her2-ihc-0/"},{"id":"her2-ihc-2-plus","kind":"biomarker","name":"HER2 IHC 2+ (equivocal, reflex to ISH)","route":"/biomarkers/her2-ihc-2-plus/"},{"id":"her2-ihc-3-plus","kind":"biomarker","name":"HER2 IHC 3+ (HER2-positive by immunohistochemistry)","route":"/biomarkers/her2-ihc-3-plus/"},{"id":"her2-low-ihc","kind":"biomarker","name":"HER2-low (IHC 1+ or IHC 2+/ISH-negative)","route":"/biomarkers/her2-low-ihc/"}],"cancer":[{"id":"breast-cancer","kind":"cancer","name":"Breast cancer (all types)","route":"/cancers/breast-cancer/"},{"id":"her2-low-metastatic-breast-cancer","kind":"cancer","name":"HER2-low and HER2-ultralow metastatic breast cancer","route":"/cancers/her2-low-metastatic-breast-cancer/"},{"id":"breast-hr-positive","kind":"cancer","name":"HR-positive / HER2-negative breast cancer","route":"/cancers/breast-hr-positive/"}],"drug":[{"id":"her2-testing-assays","kind":"drug","name":"HER2 IHC and ISH companion assays (HercepTest, PATHWAY 4B5, HER2 Dual ISH)","route":"/drugs/her2-testing-assays/"},{"id":"trastuzumab-deruxtecan","kind":"drug","name":"Trastuzumab deruxtecan","route":"/drugs/trastuzumab-deruxtecan/"}],"term":[{"id":"her2-low","kind":"term","name":"HER2-low and HER2-ultralow","route":"/terms/her2-low/"},{"id":"ihc","kind":"term","name":"Immunohistochemistry (IHC)","route":"/terms/ihc/"}],"target":[{"id":"her2","kind":"target","name":"HER2","route":"/targets/her2/"}]}}